That would put the engineering and manufacturing development (EMD) phase of the program — which includes four flying gas stations — on track to cost around $5.2 billion.

A Boeing source confirmed that figure accurately reflects a recent program cost projection following several media reports about it.

Boeing's most recent cost projection is a bit lower than 6 percent more than $4.9 billion, Assad said during a breakfast with reporters sponsored by the Center for Media and Security.

Under terms of the contract for the first phase, Boeing will absorb all costs above $4.9 billion, Pentagon and company brass say.

Boeing won the $35 billion, 179-plane contract earlier this year, beating out rival Airbus in a bitter competition that had gone on for a decade. Boeing officials have said their bid was aggressive, with Pentagon officials making it clear the Chicago-based firm’s bid was much lower than that of its European rival.